The present invention relates to a substrate holder for holding a substrate to be polished, such as a semiconductor wafer or a liquid crystal substrate, for use in a chemical/mechanical polishing (CMP) process to planarize the surface of the substrate. The present invention also relates to a method for polishing a substrate by pressing the substrate against a polishing pad and to a method for fabricating a semiconductor device by utilizing the CMP technique.
For the last decade since 1990, the diameters of CMP-processable substrates of various types, such as semiconductor wafers and liquid crystal substrates, have continued to increase. In particular, the diameter of a semiconductor wafer used to be 20-plus centimeters, but has recently reached 30 cm. On the other hand, single-wafer polishing is going to replace multi-wafer polishing day by day. Also, the feature size of a pattern formed on a semiconductor wafer has been drastically reduced over the past few years to 0.5 .mu.m or less. Under the circumstances such as these, it has become more and more necessary to planarize a semiconductor wafer, for example, even more uniformly by the polishing process.
Hereinafter, a conventional substrate polisher for use in a CMP process and a method for polishing a substrate will be described with reference to FIG. 12.
FIG. 12 illustrates an overall arrangement for a conventional substrate polisher. As shown in FIG. 12, a platen 1a, made of a rigid material with a flat surface, is attached to the top of a drive shaft 1b extending vertically downward from the lower surface of the platen 1a. The platen 1a and the drive shaft 1b are driven by a motor (not shown). An elastic polishing pad 2 is attached to the upper surface of the platen 1a. Polishing slurry 4 is supplied through a slurry supply tube 3 during polishing by a predetermined amount.
A substrate holder 100 for holding a substrate 5 to be polished thereon is provided over the polishing pad 2. The substrate 5 is pressed against the pad 2 while being rotated by the substrate holder 100.
In this polisher, the pad 2 is rotated along with the platen 1a, while the substrate 5, held by the substrate holder 100, is also rotated and pressed against the pad 2 with the slurry 4 supplied through the slurry supply tube 3 onto the pad 2. As a result, the surface of the substrate 5 to be polished continuously receives pressure from the polishing pad 2 at a certain relative velocity and is polished.
If the surface of the substrate 5 to be polished has some roughness, then such roughness can be reduced through this polishing process and the substrate 5 has its surface planarized. This is because convex portions of the surface are more likely to be polished since the contact pressure between those portions and the pad 2 is relatively high and the relative polishing rate also increases. On the other hand, concave portions thereof are hardly polished because the concave portions hardly come into contact with the polishing pad 2, or should such contact happen, the resulting contact pressure therebetween is relatively low.
As described above, when chemical/mechanical polishing is carried out, the entire surface of a semiconductor wafer should be polished and planarized even more uniformly recently. To meet this demand, a substrate holder such as that disclosed in Japanese Laid-Open Publication No. 8-339979 was proposed.
FIG. 13 illustrates a substrate holder 100A according to a first prior art example disclosed in Japanese Laid-Open Publication No. 8-339979 identified above. As shown in FIG. 13, the substrate holder 100A for holding a substrate 5 to be polished thereon and pressing the substrate 5 against a polishing pad 2 is disposed over the polishing pad 2 attached to the upper surface of a platen 1a.
The substrate holder 100A according to the first prior art example includes a drive shaft 101, a disklike substrate-holding head 102, a ringlike seal member 103 made of an elastic body and a ringlike guide member 104. The substrate-holding head 102 is integrated with the drive shaft 101 at the lower end thereof. The seal member 103 is secured to the lower surface of the substrate-holding head 102 in a peripheral region thereof. And the guide member 104 is secured around the outer periphery of the seal member 103 on the lower surface of the substrate-holding head 102. A fluid path 105 runs through the drive shaft 101 and the substrate-holding head 102. Pressurized fluid or air is introduced through the upper end of the fluid path 105, passed through the lower end of the path 105 and then supplied into a space 106, which is formed by the substrate-holding head 102, seal member 103 and substrate 5. The pressurized fluid, which has been supplied into the space 106, presses the substrate 5 against the polishing pad 2. As a result, the substrate 5 is polished.
FIG. 14 illustrates a substrate holder 100B according to a second prior art example. As shown in FIG. 14, the substrate holder 100B includes a drive shaft 101, a disklike substrate-holding head 102, a back pad 108 made of an elastic body and a ringlike guide member 104. The substrate-holding head 102 is integrated with the drive shaft 101 at the lower end thereof. The back pad 108 is secured to the lower surface of the substrate-holding head 102. And the guide member 104 is secured around the outer periphery of the back pad 108 on the lower surface of the substrate-holding head 102. In this configuration, if the pressure inside a fluid path 105, which runs through the substrate-holding head 102, is reduced, then the substrate 5 is held tight on the substrate-holding head 102. On the other hand, if the pressure inside the fluid path 105 is increased, then the substrate 5 is released from the substrate-holding head 102. Also, when the substrate-holding head 102, which is holding the substrate 5 thereon, is pressed against the polishing pad 2, the substrate 5 is polished.
FIGS. 15(a) and 15(b) illustrate a relationship between the distance from the center of the substrate and the polishing rate where the substrate is polished using the substrate holders according to the first and second prior art examples. As can be seen from FIGS. 15(a) and 15(b), the polishing rate abruptly increases in the outer edge region of the substrate 5.
The present inventors carried out intensive research to find out why the polishing rate abruptly increased in the outer edge region of the substrate 5 that had been polished using the substrate holders according to the first and second prior art examples. As a result, we reached the following conclusion.
As shown in FIG. 16, during the polishing process of the substrate 5, the substrate-holding head 102 is rotating in the direction as indicated by the arrow B while being pressed downward against the polishing pad 2 as indicated by the arrow A. Accordingly, the polishing pad 2, which is usually made of an elastic body such as foamed polyurethane or a non-woven fabric, receives the forces applied in the respective directions A and B from the substrate 5 and the guide member 104. Thus, portions of the polishing pad 2, which come into contact with the respective outer edges of the substrate 5 and the guide member 104, are likely to form protrusions 2a and 2b. And the respective regions near the outer edges of the substrate and the guide member 4 receive high pressure as a result of rebounding of the protrusions 2a and 2b of the polishing pad 2 as indicated by arrows C representing rebounding force in FIG. 16. It is probably because the region near the outer edge of the substrate 5 receives much pressure from the protrusion 2a that the polishing rate in the outer edge region of the substrate 5 is a lot higher than that in the center region of the substrate 5.
Thus, a proposed substrate holder includes respective cylinders for pressing the seal and guide members 103, 104 to make the pressure applied to the seal member 103 (and to the outer edge region of the substrate 5) lower than that applied to the guide member 104. In the substrate holder, the pressure applied to the guide member 104 is set higher than that applied to the seal member 103. In this manner, it is possible to prevent the polishing pad 2 from forming the protrusion 2a around the outer edge of the substrate 5, thereby equalizing the polishing rate over the entire surface of the substrate 5.
However, if the pair of cylinders for pressing the seal and guide members 103, 104 are provided separately, then two systems of pressurizing mechanisms are needed, thus adversely complicating the structure of the substrate holder.